Experience with return voltage measurements for assessing insulation conditions in service-aged transformers

This paper describes the investigation of the insulation condition of a number of power and distribution type transformers of different manufacturing dates and with different operating histories. Return voltage measurements were conducted for these transformers. Effects of moisture and aging on the oil-paper insulation of these transformers were investigated and results are presented in this paper. Moisture in oil samples at known temperature was measured for these transformers. Previously accelerated aging experiments were performed on paper wrapped insulated conductors in different environments over a temperature range of 115 to 145/spl deg/C. Results from the measurements on transformers are compared with those of the accelerated aged samples and are described in this paper.

I. Introduction

In RECENT years, there has been a growing interest in the condition assessment of transformer insulation. Primarily this is due to the increasingly aged population of transformers in utilities around the world. A large proportion of existing power transformers within electric utilities are approaching the end of their design life. Many, perhaps most, seem to be operating satisfactorily. However, insulation degradation continues to be a major concern for these aged transformers. Insulation materials degrade at higher temperatures in the presence of oxygen and moisture. The degradation from thermal stress affects electrical, chemical, and mechanical properties [9].